Laser systems are used for active imaging in degraded visual environments. These systems transmit a laser pulse that is reflected off a target object in the environment, which is then received by an optical detector. The detected laser reflection is processed, either through digital or analog processing, to create an image of the target object. Conventional laser imaging systems suffer from degraded visuals due to the collection of scattered light. The collection of scattered light is the result of multiple sources. First, some light is scattered in the backward direction and never reaches the object of interest. This backscattered light decreases image contrast and increases the noise level at the receiver. Second, light scatters at small angles on its path to the object, the once collimated laser beam becomes spatially dispersed and illuminates parts of the object outside of the area of interest. The small angle light scatter is termed ‘blur-glow scatter’ since it causes image blurring and loss of spatial resolution. Last, forward-scattered light can reach object features—and/or the background—that may be at different ranges. The collection of each of these light scatter sources can cause each illuminated pixel to contain contributions from other parts of the object. Where systems cannot resolve the returns at different ranges, errors in range estimations can occur.
Light that is backscattered on its path to the target object can be significantly reduced by using a pulsed laser (1-2 ns pulse width) and a high speed, range-gated receiver. In some instances, the receiver can be timed to ‘open’ at a time corresponding to the roundtrip delay to the object. Therefore, the receiver is ‘closed’ when the early-arriving backscattered signal appears at the receiver and is suppressed. However, the system is blind to objects that are located outside of the gate ‘on’ time, which could lead to error. Furthermore, gating can cause a highly sensitive photo receiver to suffer from ringing due to impedance mismatches.
An approach that combines radar modulation, demodulation, and signal processing schemes with a pulsed time of flight laser system was developed as an alternate to the conventional short pulse approach. This technique uses a high speed (>100 MHz) narrowband-modulated 20-30 ns laser pulse and a high speed, matched filter receiver. The signal reflected from an object retains its modulation and can be recovered at the receiver. By bandpass filtering the detected signal at the modulation frequency, the backscatter is suppressed and the object-reflected signal is enhanced.
Multiple forward scattering causes a short optical pulse to be stretched in time. Since the range resolution is directly proportional to the laser pulse width, temporal stretching due to scattering reduces the achievable range resolution. Small angle multiple scattering becomes uncorrelated with the initial pulse, allowing the narrowband modulated pulse approach to filter out forward-scattered light. However, in some cases, the range resolution is not sufficient to distinguish between two, closely spaced objects or different range features of a three dimensional object. Furthermore, if the object dimensions are on the same order as the wavelength of the modulation signal, returns from different parts of the object can constructively and destructively interfere which introduces distortions in the collected imagery. Despite the breadth of advances made in optical imaging of degraded visual environments, image quality remains an issue.